This invention relates to a sub-band notification method and terminal apparatus, and more particularly to a sub-band notification method and terminal apparatus of a cellular system that performs frequency domain scheduling based on the radio link quality of the downlink, and adaptively assigns a sub band to each terminal.
In an EUTRAN (Evolved UTRAN) system (see 3GPP TR 25.814 V7.0.0 (2006-6)) that is being studied as a next-generation type 3GPP system, scheduling is performed for both the downlink from a base station to a mobile station and the uplink from a mobile station to the base station, in the frequency domain as well as in the time domain. In other words, the transmission bands for both the downlink and uplink are divided into a plurality of sub bands, and basically, the sub bands having the best quality are assigned to the terminals (mobile stations) and data is transmitted.
In the downlink, a predetermined common pilot is transmitted from the base station at a fixed period over the entire transmission band. FIG. 17 shows an example of a radio frame in the downlink, and in each sub band, after each set period, or in other words after each TTI (Transmission Time Interval), a common pilot, control signal, user data and the like are transmitted. The control signal contains user ID information that indicates which terminal apparatus (mobile station) the user data transmitted in the TTI is for. By decoding the control signal and checking that user ID information, each terminal is able to determine whether or not it decodes the following user data that is also contained in that same TTI.
As shown in FIG. 18, by performing radio measurement (SNR or SIR measurement) of the common pilots (pilot #1 to pilot #60) for each sub band, each terminal MS is able to know which band has the best quality for the terminal. Each terminal MS reports all or part of the radio measurement results as is, or converts the radio measurement results into a different format and reports the results to the base station BS. By doing so, it becomes possible for the base station BS to transmit individual data (dedicated data) to the terminals using sub bands having the best quality for each respective terminal MS (scheduled transmission by the base station). Various methods have been considered as the method used by the terminal MS to designate the format of the measured results and to report the results to the base station BS, however, a typical method is a method as shown in FIG. 18 in which the ID number of the sub-band having the very best quality and the radio measurement results are transmitted to the base station BS in the uplink.
In EUTRAN, it is assumed that both the downlink and uplink have a maximum of a 20 MHz transmission bandwidth as shown in FIG. 17. The bandwidth of the sub bands in the downlink is assumed to be in the range of 300 kHz to 400 kHz, and the number of sub bands in a transmission band having a transmission bandwidth of 20 MHz is about 50 to 60 bands. When an ID number is assigned to a sub band, the length of the signal indicating that ID number is 6 bits when the addition of CRC (cyclic redundancy check) code and code processing are not performed.
Generally, the data transmission characteristics in the downlink improves as the frequency of reporting the measurement results from the terminal MS to the base station BS increases. This is because the amount of time in which the optimum sub band is used for transmitting data to the terminal increases. In other words, this is because when transmitting data to the terminal, the probability that the optimum sub band will be used increases. When a sub band having good quality is used, it becomes possible to perform data transmission at a high data rate, and as a result, the data throughput characteristics improve. However, the higher the frequency that measurement results are reported from the terminal MS to the base station BS, the radio resource consumption in the uplink increases. Taking into consideration the radio resource consumption in the uplink, the frequency of reporting measurement results is decreased, however, that causes degradation of the downlink data transmission characteristics.
As prior art there is a multi-frequency-load-sharing method in a mobile communication system (see Japan patent application 2004-15697). This multi-frequency-load-sharing method makes it possible to share the load of a plurality of frequencies by assigning and using a frequency according to the state of use of all of the frequencies in response to a request from a call-control-processing unit. However, in this prior art, the base station does not perform frequency-domain scheduling based on the radio link quality of the downlink which is reported from the terminal, and does not adaptively assign a sub band for each terminal. Moreover, the base station does not suppress an increase in radio resource consumption in the uplink, even though the frequency of reporting measurement results of the radio link quality to the base station from the terminal is increased.
When transmitting data from a base station to a terminal, the quality of the sub band used for transmitting the data changes over time, however, it is often the case that the quality of a sub band adjacent to or near the current sub band being used becomes better. However, when it is desired to use that adjacent sub band, using 6 bits to report the ID number of that sub band is not preferable when radio resource consumption of the uplink is considered. If it were possible to effectively reduce the number of bits used for specifying the sub band, it would not be necessary to reduce the frequency of reporting measurement results from the terminal to the base station, and it would become possible to improve the data transmission characteristics of the downlink.